1. Field of the Invention
The present invention is generally related to the field of semiconductor processing, and, more particularly, to a method and apparatus for controlling the amount of light energy delivered to a layer of photoresist on a semiconductor device.
2. Description of the Related Art
In general, semiconductor devices are manufactured by forming many process layers comprised of various materials above a semiconducting substrate, and, thereafter, removing selected portions of the layers, i.e., patterning the layers. This patterning may be accomplished using known photolithography and etching processes to define the various features of the device, e.g., a gate insulation layer, a gate electrode, metal lines and contacts, etc. This forming and patterning of the process layers is typically performed layer by layer as the individual layers are formed, although multiple layers may be patterned at any given time.
Photolithography is a common process used in patterning these various layers. Photolithography typically involves the use of a product known as photoresist. In general terms, photoresist is a product that may be changed from a relatively soluble state to a relatively insoluble state by exposure to a light source. There are positive and negative photoresists currently available on the market.
The photolithography process generally involves forming a layer of photoresist above a previously formed process layer, and exposing selected portions of the layer of photoresist to a light source to form a pattern in the photoresist. The pattern formed in the photoresist is subsequently transferred to the underlying process layer. All of these steps are typically performed in well-known photolithography modules that include a section for depositing the photoresist on the wafer, e.g., a spin-coating station, a device for selectively exposing portions of the photoresist layer to a light source through a reticle or photomask, e.g., a stepper, and a section for rinsing and developing the photoresist layer after it has been selectively exposed to the light source. Thereafter, an etching process, such as a plasma etching process, is performed to remove portions of the underlying process layer that are not covered by the patterned layer of photoresist, i.e., the patterned layer of photoresist acts as a mask. After the etching process is complete, the patterned photoresist layer is typically removed so that additional process layers may be formed above the now patterned process layer.
The purpose of the photoresist application step is to form a thin, uniform, defect-free layer of photoresist above the substrate surface. Typically, the photoresist is developed by exposing it to a light source through an optical system for a preselected duration of time. Ordinarily, the optical system has a number of parameters, such as numerical aperture, focus, coherence, and the like. Often, these parameters are adjusted to a desired setting, which is intended to produce a suitable image on the layer of photoresist. Over time, however, outside factors, such as environmental conditions, may cause the settings to drift or render them less effective. For example, variations in barometric pressure, temperature, and the like may result in the current focus no longer producing the desired image on the layer of photoresist.
Poor focus may have undesirable effects on the developed layer of photoresist. That is, dimensions of the patterns formed in the photoresist may be affected by other than ideal focus. This dimensional variation may carry over to the features that are to be formed in the semiconductor device, and, thus, affect the operation of the semiconductor device, or in the worst case render it inoperable.
The present invention is directed to a method of solving, or at least reducing the effects of, some or all of the aforementioned problems.
In one aspect of the present invention, a method is provided. The method is comprised of energizing a light source. A temperature of a lens used to focus the light source is determined, and then a desired focus of the lens based upon the temperature of the lens is determined. The focus of the lens is set at the desired focus, and then a device is exposed to the light source through the lens.
In another aspect of the instant invention, a system is provided. The system comprises a stepper, an optical system, and a controller. The stepper has a light source controllably energizable to provide light to a surface of a semiconductor device. The optical system has a lens with a controllable focus. The controller is capable of determining a temperature of the lens, and controllably varying the focus of the lens in response to the temperature of the lens.
In yet another aspect of the instant invention, a method is provided. The method is comprised of energizing a light source. The light source is exposed to a device through a lens, wherein the lens is set to a first preselected focus. A parameter of the device is measured, and a second preselected focus is determined based on the measured parameter. The focus of the lens is set to the second preselected value for processing of subsequent devices.